1. Field of The Invention
This invention relates to a gain and phase control system for adjusting the relative phase and gain between combining signals, for example in a distortion reduction system.
2. Description of Related Art
Amplifiers often add undesired distortion to a signal, creating an output signal comprising distortion or nonlinear components and the signal component. The distortion includes any undesired signals added to or affecting adversely the input signal. There is therefore a need to devise techniques that can eliminate substantially or reduce significantly the distortion produced by the amplifier.
Feed-forward correction is routinely deployed in modern amplifiers to improve amplifier linearity with various input patterns. The essence of the feed-forward correction is to manipulate distortion, such as intermodulation (IMD) components, created by the amplifier so that at the final summing point, the distortion cancels out. Due to the unpredictability of input RF carrier pattern as well as the resultant distortion location, a known frequency component, i.e. a pilot signal, is injected in the main signal path with the distortion produced by the amplification process. In feed-forward amplifiers, the feed forward distortion reduction circuitry minimizes the pilot signal along with the distortion. As such, by designing the feed forward distortion reduction circuitry to detect and cancel the pilot signal, the distortion can also be removed.
The pilot signal is an electrical signal comprising at least one frequency component spectrally located near the frequency band of operation of the electrical circuit. A more complete description of the pilot signal is shown in FIG. 1 which shows the frequency response of a radio frequency (RF) amplifier including the location of the pilot signal. The pilot signal can be near the lower edge of the operating band (e.g., pilot 1) and/or located near the upper edge of the band of operation (e.g., pilot 2). The pilot is positioned a spectral distance of xcex94f from an edge of the band of operation whose center frequency is f0. The electrical characteristics (e.g., amplitude, phase response, spectral content) of the pilot signal are known. It should be noted that although the pilot signal is shown as having one or two spectral components of a certain amplitude, the pilot signal can comprise a plurality of spectral components having various amplitudes.
The feed forward distortion reduction circuitry reduces distortion produced by the RF amplifier by applying the pilot signal to the RF amplifier and making adjustments based on information obtained from the applied pilot signal. FIG. 2 discloses feed-forward correction circuitry 10 and its use of information obtained from the pilot signal to reduce distortion produced by RF amplifier 12. An input signal, for example including at least one carrier signal, is applied to a splitter 14. The splitter 14 replicates the input signal on a main signal path 16 and a feed forward path 18. The splitter 14 is part of a carrier cancellation loop referred to as loop #1, which in addition to the splitter 14, comprises gain and phase circuit 20, coupler 22, the RF amplifier 12, delay circuit 24 and couplers 26 and 28. The signal on the main path 16 is applied to gain and phase circuit 20. The output of gain and phase circuit 20 and the pilot signal are applied to the coupler 22. Typically, the amplitude of the pilot signal is much less (e.g., 30 dB less) than the amplitude of the input signal so as not to interfere with the operation of the amplifier 12. The output of coupler 22 is applied to the amplifier 12 whose output comprises the amplified input signal, the amplified pilot signal and distortion signals produced by the amplifier 12.
A portion of the output of the amplifier 12 is obtained from the coupler 26 and is combined at the coupler 28 via coupling path 30 with a delayed version of the input signal on the feed forward path 18 to isolate the pilot signal with distortion on the feed forward path 18. The input signal on the feed forward path 18 is sufficiently delayed by delay circuit 24 so that such signal experiences the same delay as the signal appearing at the coupler 28 via the path 30. The resulting error signal contains the distortion produced by the amplifier 12 along with any portion of the carrier signal remaining at the output of the coupler 28 and the pilot signal. The amount of carrier cancellation in the carrier cancellation loop depends on the proper gain and phase match between the two paths from the splitter 14 to the coupler 28.
The gain and phase circuit 20 adjusts the phase and gain of the input signal according to control signals on control paths 32 and 34 such that the signal appearing at the coupler 28 via the path 30 is substantially the inverse (equal in amplitude but 180xc2x0 out of phase) of the delayed input signal at the coupler 28. The gain and phase control signals appearing on the control paths 32 and 34 of the gain and phase circuit 20 are derived from the signal at the output of the coupler 28 in a well known manner using signal detection and control circuitry 35. In general, the signal detection and control circuitry 35 detects an error signal for the carrier cancellation loop. The error signal represents the amplitude of the signal at point A, and the signal detection and control circuitry 35 attempts to reduce the amplitude of the error signal by providing gain and/or phase control signals.
In this embodiment, the signal detection and control circuitry 35 includes a detector 36, such as a log detector, which produces a signal representing the amplitude of the signal at point A. A filter 38 filters the output of the log detector to produce a DC-type amplitude signal representing the amplitude of the error signal. The amplitude signal is provided to a nulling circuit 40. In response to the amplitude signal, the nulling circuit 40 provides the control signals on the control paths 32 and 34 to reduce the error signal, thereby reducing the carrier signal(s). When the error signal is minimized, the carrier signals combined at the coupler 28 substantially cancel each other leaving at the output of the coupler 28 the pilot signal with distortion produced by the amplifier 12. Loop #1 is thus a carrier cancellation loop which serves to isolate on the feed forward path 18 the pilot signal with distortion produced by the amplifier 12.
A distortion reduction loop or loop #2 attempts to reduce the pilot signal on the main signal path 16, thereby reducing the distortion produced by the amplifier 12, using the error signal at the output of the coupler 28. The pilot signal with distortion on the feed forward path 18 is fed to a gain and phase circuit 42. The output of the gain and phase circuit 42 is fed to amplifier 44 whose output is applied to coupler 46. The coupler 46 combines the amplified pilot signal and distortion on the feed forward path 18 with the signals from the amplifier 12 on the main signal path 16 (carrier signal(s), pilot signal with distortion). A delay circuit 40 on the main signal path 16 delays the signals from the output of the amplifier 12 on the main signal path 16 to experience substantially the same delay as the corresponding signals from the output of the amplifier 12 which pass over the coupling path 30 through the coupler 28 to the coupler 46.
A coupler 48 provides an error signal representative of the signal at the output of the coupler 46 onto a pilot detection path 50. Because the frequency, amplitude and other electrical characteristics of the pilot signal are known, pilot detection and control circuitry 52 can detect the amplitude of the remaining portion of the pilot signal from the error signal on the pilot detection path 50. The pilot detection and control circuitry 48 determines the amplitude of the pilot signal, and in response to the amplitude of the remaining pilot signal, the pilot detection and control circuitry 52 provides control signals to the phase and gain circuit 42. In general, the pilot detection and control circuitry 48 will detect the pilot signal and use this information to generate control signals onto paths 66 and 68 to cause the gain and phase circuit 42 to modify the pilot signal on the feed forward path 18 such that the pilot signal on the main path 16 is substantially the inverse (equal in amplitude but 180xc2x0 out of phase) of the pilot signal on the feed forward path 18 at the coupler 46. The corresponding pilot signals and distortion substantially cancel each other at the coupler 46 leaving the carrier signal(s) at the output of the system. Therefore, loop #2 is a distortion reduction loop which attempts to cancel the pilot signal to cancel substantially the distortion produced by the amplifier 12.
In this embodiment, the pilot detection and control circuitry 52 includes pilot receive circuitry 54 which includes a mixer 56 to frequency convert the error signal on the pilot detection path 52 to lower frequencies and a filter 58 to facilitate detection of the pilot signal by a signal detector 60. The detector 60, such as a log detector, produces a signal representing the amplitude of the signal at point B. A filter 62 filters the output of the detector 60 to produce a DC-type amplitude signal representing the amplitude of the remaining pilot signal or error signal. The amplitude signal is provided to a nulling circuit 64. In response to the amplitude signal, the nulling circuit 64 provides control signals on the control paths 66 and 68 to the phase and gain circuit 42. The control signals are provided to reduce the amplitude signal, thereby reducing the remaining pilot signal. The amount of cancellation of the pilot signal indicates the amount of distortion cancellation. When amplitude of the pilot signal is minimized, the pilot signals and distortion combined at the coupler 46 substantially cancel each other at the output of the coupler 46.
In actual systems, however, there is rarely an absolute cancellation of the combining signals. The amount of signal cancellation depends on the proper gain and phase match between the combining signals. Signal reduction as a function of gain and phase mismatch is shown in FIG. 3. The gain and phase characteristics of the amplifiers 12 and 44 as well as of the other devices vary over time. Such variations are typically due to the temperature, input power, device age and manufacturing variations. To maintain carrier cancellation performance in the carrier cancellation loop and distortion reduction in the distortion reduction loop, the signal detection and control circuitry 35 and the pilot detection and control circuitry 52 are designed to automatically control the gain and phase characteristics for the corresponding carrier cancellation and distortion reduction loops based on the amplitudes of the corresponding error signals.
The nulling circuits 40 and 64 attempt to reduce the error signal (indicating improved cancellation) by comparing every error signal sample with a previous error signal sample. In this embodiment, the pilot detect and control circuitry 52 uses the pilot signal as a reference error signal. In this embodiment, each nulling circuit makes two types of adjustments (gain and phase) based on the amplitude signal from one detector 36 or 60, and the nulling circuit 40 or 64 performs a series of n steps of phase adjustments (for example 12 steps) in sequence with n steps of gain adjustments. This alternating series of gain and phase adjustments is repeatedly performed to reduce the error signal toward null. The nulling circuit 40 or 64 determines how to adjust the phase or gain adjustment value (for example Vout(n+1)) based on the results of the comparison between an error signal sample resulting from the last adjustment (for example error(n)) and the previous error signal sample (for example, error(nxe2x88x921)). However, because gain and phase are relatively independent of each other, an incorrect gain or phase adjustment can be made after a series of n phase or gain adjustments. For example, as shown in FIG. 4, the magnitude of the error signal decreases after a 12 steps 80a-l of gain adjustments, and the nulling circuit makes a phase adjustment Vphase(n+1) based on the previous error signal samples after steps 80k and 80l resulting from gain adjustments (for example, errorgain(n)xe2x88x92errorgain(nxe2x88x921). About half the time, the nulling circuit will make an incorrect phase adjustment based on the results of the last two gain adjustments, resulting in a jump 82 in the error signal. After the jump 82, steps 84a-l of phase adjustments bring the error signal back down, but the improper adjustment determination adversely effects the convergence rate.
The nulling sensitivity and the location of the null varies as system parameters and transmit power varies. For example, in multi-user wireless communications systems, such as Code division multiple access (CDMA), Time division multiple access (TDMA), Global System for Mobile Communications (GSM) and orthogonal frequency division multiplexing (OFDM), multiple voice and/or traffic channels are combined into a single or multiple carriers. A linear amplifier should be able to react rapidly to transmit power changes and bursty traffic variations within the transient response specifications in the microsecond and millisecond ranges while providing adequate error cancellation.
The present invention involves a gain and phase control system which performs a gain (or phase) adjustment based on the results of a previous gain (or phase) adjustment after an intervening phase (or gain) adjustment. A gain adjustment is based on the results of a previous gain adjustment rather than on the results of an intervening phase adjustment. A phase adjustment is based on the results of a previous phase adjustment rather than the results of an intervening gain adjustment. As such, the above-mentioned increase in error signal does not occur because a gain adjustment is based on the results of the previous gain adjustment, and a phase adjustment is based on the results of the previous phase adjustments. In accordance with another aspect of the present invention, the gain and phase control system makes a gain (or phase) adjustment in parallel with detecting the results from a previous phase (or gain) adjustment, thereby taking advantage of the delay between adjustment and detection of the resulting error signal to improve the convergence rate. For example, the gain and phase control system can perform single alternating gain and phase adjustments and make a gain (or phase) adjustment while detecting the error signal resulting from a preceding phase (or gain) adjustment. When the error signal resulting from the gain (or phase) adjustment is being detected, the gain and phase control system makes a phase (or gain) adjustment using the results detected from the preceding phase (or gain) adjustment.